Publication Date

2012

Document Type

Honors Thesis

Department

Geosciences

Keywords

Petrogenesis-Iceland, Geochemistry-Iceland, Rifts (Geology)-Iceland, Petrology-Iceland, Geology, Mid-ocean ridge, Plume, Basalt, Dacite, Mineralogy, Petrography, Ophitic, Iceland

Abstract

The Hrafnfjordur central volcano in northwestern Iceland formed in the early stages of development of the Skagi-Snaefellsnes rift zone approximately 14 million years ago, before the rift drifted away from the mantle plume. This project aims to investigate the petrogenesis of rocks associated with Hrafnfjordur to characterize this stage in rift development. The rocks in this area consist of basalts, basaltic andesites, andesites and dacites. The basalts were categorized into two distinct groups based on petrographic and geochemical evidence. The more prevalent type exhibits an intergranular-intersertal texture and enriched MORB chemistry. E-MORB composition is typical in Icelandic rift zone basalts due to the influence of the mantle plume beneath the rift. The less common type displays an ophitic texture and is depleted in incompatible elements relative to the other basalts. This variation in chemistry is likely due to derivation of the ophitic basalts from a depleted mantle plume source, which would also account for the difference in petrographic texture. Geochemical data strongly suggest that the non-ophitic, E-MORB basalts are related to one another and also to the andesites through fractional crystallization of plagioclase, clinopyroxene, olivine, ilmenite, magnetite and apatite. The basaltic andesite resembles the non-ophitic basalts in thin section except that its plagioclase phenocrysts exhibit striking resorption and regrowth textures. These textures, along with the fact that the basaltic andesite does not fit on fractional crystallization trends with the basalts and andesites, suggest a history of magma mixing. Various basaltic and silicic rocks from this field area and nearby localities were evaluated using modeling in an attempt to identify the mixing end members. However, these attempts were unsuccessful, and no conclusive ii evidence was found to indicate that any of the available samples represented an end member in this mixing reaction. Lastly, the dacites in this area do not fit along fractional crystallization trends with the basalts and andesite. They also have an anomalously high Zr/Nb ratio, and their plagioclase phenocrysts are significantly more calcic than those in the andesite, suggesting that the dacites probably are not related to the basalts and basaltic andesites through fractional crystallization. The other recognized process by which silicic magma is produced is partial melting of the crust, and it is likely that this process explains the dacite production in this area. The data support the interpretation that basaltic rocks in the Hrafnfjordur region formed from two separate plume sources (enriched and depleted), and that the more silicic rocks formed through fractional crystallization, magma mixing, and partial melting of the crust. These interpretations suggest that the signature of a depleted plume source and partial melting of the crust are characteristic of magmas from the early stages of Icelandic rift zones, as can be seen in the neovolcanic zone located directly above the rift today.

Language

English

Comments

vi, 69 p. : col. ill., col. maps. Honors project-Smith College, Northampton, Mass., 2012. Includes bibliographical references (p. 68-69)

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